1. Field of the Invention
This invention relates generally to current mirror circuitry and more particularly to a current mirror circuit for an integrated circuit which is not affected by changes in temperature.
2. Discussion of the Background
Many forms of current mirror circuits have been developed, many of which have been used in integrated circuitry. It is an important requirement for such circuits that they not be effected by variation in the circuit temperature. Many of the prior art devices have not been suitable due to the effect of a change in temperature on the circuit output. Since current mirror circuits in integrated circuitry are often used in portable equipment or miniature electronic devices they often are required to operate with a low power supply voltage. In fact, some present portable equipment utilizes a single dry cell of 11/2 volts as a power supply. Since this power supply will diminish as the battery gets old, it is desirable to have the circuit be operable even if the voltage of the battery drops to 0.9 volts.
One type of current mirror circuit known in the prior art is shown in FIG. 1 as including a pair of transistors 110 and 112 having their base-emitter paths connected in parallel. The bases of the two transistors are directly connected to each other and also to the collector of transistor 110. The emitters of the two transistors are both connected to ground.
The collector of transistor 110 is coupled to the collector of transistor 114. Transistor 114, resistor 116 and signal source 118 constitute an input current source. The collector of transistor 112 is connected to an output terminal.
This prior art circuit presents problems in that it does not work well where low voltages are possible. In particular, the transistor 114 will not remain operational since the emitter voltage and the collector voltage are both equal to the base-emitter offset voltage of about 0.6 volts when using a transistor with a low bias current. This limitation would preclude the operation of the circuit in low voltage circumstances.
An improved current mirror circuit is disclosed in Japanese laid-open disclosure No. 60-33717 which is assigned to the assignee of the present application. This improved current mirror circuit uses an additional current source and resistor and is operated with a low supply voltage. However, this circuit does not react well to variations in the temperature. Generally, this type of circuit is required to operate in a range of temperatures of -25.degree. C. to 75.degree. C. If the temperature reaches either extreme of 75.degree. C. or -25.degree. C., a transistor in this current mirror circuit will saturate and not operate properly. Furthermore, the input node of the circuit is coupled to a resistor, whose resistance may vary in the course of production of the device, thus causing the potential at the input node not to stabilize.